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The Stem Crustacean Oelandocaris Oelandica Re−Visited

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The Stem Crustacean Oelandocaris Oelandica Re−Visited The stem crustacean Oelandocaris oelandica re−visited MARTIN STEIN, DIETER WALOSZEK, ANDREAS MAAS, JOACHIM T. HAUG, and KLAUS J. MÜLLER Stein, M., Waloszek, D., Maas, A., Haug, J.T., and Müller, K.J. 2008. The stem crustacean Oelandocaris oelandica re−visited. Acta Palaeontologica Polonica 53 (3): 461–484. The arthropod Oelandocaris oelandica from the upper Middle Cambrian “Orsten” of Sweden was recently recognized as a member of the early phase of crustacean evolution based on additional morphological detail from new specimens. Here we present a detailed investigation of all available material. It includes the description of a 400 μm long specimen proba− bly representing an early developmental stage. Variation in size correlated with variation of trunk−segment numbers al− lowed recognition of different instars. The largest specimens do not exceed an estimated length of about 1 mm, indicating that our material may consist only of immature specimens. The characteristic, extremely long antennula of O. oelandica branches into three long rods. It may have served as the major structure to sweep in food, aided by the two subsequent ap− pendages. These and the more posterior limbs were also responsible for locomotion. Minute pores on the outer edges of the posterior limbs and on the trunk tergites possibly contained sensilla originally, which may have served as water−cur− rent detectors. The presence of a minute proximal endite only on the third head appendage suggests a rather basal position of this species within Crustacea, because comparable developmental stages of other known stem crustaceans have such an endite on more of their appendages. Reconstruction of O. oelandica and its life attitudes (referred to the largest instar known) benefited from the application of 3D modelling. These helped, e.g., in identifying the combination of the plesiomorphic feeding function of the antennulae and the specialisation of the exopods of the next two appendages as a step toward the development of a sweep−net mode of feeding, one of the key novelties in the evolution of Crustacea. Such a mode of feeding coupled with locomotion of the three anterior appendages is still practiced in the naupliar and metanaupliar phases of many extant eucrustaceans, and even some adults. Key words: Crustacea, Arthropoda, morphology, life habits, sweep−net feeding, evolution, phylogeny, stem lineage, computer−aided 3D modelling. Martin Stein [[email protected]], Department of Earth Sciences (Palaeobiology), Uppsala University, Villavägen 16, 75236 Uppsala, Sweden; Dieter Waloszek [dieter.waloszek@uni−ulm.de], Andreas Maas [andreas.maas@uni−ulm.de], and Joachim T. Haug [joachim.haug@uni−ulm.de], Biosystematic Documentation, University of Ulm, Helmholtzstrasse 20, 89081 Ulm, Ger− many; Klaus J. Müller, Institute of Palaeontology, University of Bonn, Nussallee 8, 53115 Bonn, Germany. Introduction hexapods) and Eucrustacea as the crown group including all taxa with extant derivatives. In 1983 Müller described six Cambrian putative crustacean Some autapomorphies traditionally proposed for Crusta− species on the basis of specimens in an unusual three−dimen− cea, e.g., differentiation of cephalic appendages into two sional type of phosphatic preservation. Subsequent study of pairs of (sensorial) antennae, mandibles and two pairs of additional arthropod taxa in this “Orsten”−type preservation maxillae, could be soundly refuted and demonstrated to be (Maas et al. 2006) led to the reconstruction of the stem lin− valid only for certain eucrustacean in−group taxa; instead, a eage of Crustacea (putative stem taxa: Cambropachycope new set of autapomorphies was proposed (Walossek and clarksoni Walossek and Müller, 1990, Goticaris longispi− Müller 1990; 1998a, b; Walossek 1999; Maas et al. 2003; nosa Walossek and Müller, 1990, Henningsmoenicaris scu− Waloszek 2003a, b). Two of the new characters are notewor− tula [Walossek and Müller, 1990], Martinssonia elongata thy in this context: Müller and Walossek, 1986; subsequently added: Cambro− – exopods of, at least, the first two post−antennular limbs caris baltica Walossek and Szaniawski, 1991) and identifi− multi−annulated and with locomotory setae arising from cation of the abundant bivalved Phosphatocopina as the sis− the inner side of the annuli facing the endopod; ter group of Eucrustacea (crown group Crustacea; Maas et al. – a small setiferous endite, called “proximal endite”, located 2003; Siveter et al. 2003; Maas and Waloszek 2005). This medially below the basipod of the post−antennular ap− helped consolidate our understanding of the ground patterns pendages (Walossek and Müller 1990). of Crustacea sensu lato, Labrophora (Phosphatocopina + Multi−annulated exopods form part of a sweep−net feed− Eucrustacea, possibly also including the myriapods and/or ing and locomotion apparatus and occur in the Cambrian Acta Palaeontol. Pol. 53 (3): 461–484, 2008 http://app.pan.pl/acta53/app53−461.pdf 462 ACTA PALAEONTOLOGICA POLONICA 53 (3), 2008 phosphatocopines and in extant small−scale eucrustaceans life attitudes. In addition we describe a small specimen, ap− and all free early eucrustacean larvae. The “proximal endite” parently of an early larval stage as the putative earliest stage had been recognized on extant branchiopods almost 90 years known of this species, filling the series of instars. earlier (Calman 1909), but remained largely ignored subse− quently. We regard its development as a significant step in Institutional abbreviation.—UB, University of Bonn, Ger− the change of the locomotory and feeding attitudes along the many. evolutionary lineage of crustaceans (see also, e.g., Waloszek et al. 2007). Further modification of the “proximal endite” on the second and third appendages, the so−called antennae and Material mandibles, into a coxa is an autapomorphy of the Labrophora (Siveter et al. 2003). Besides these features, the Labrophora All material was etched from bituminous limestone nodules can be distinguished from the stem taxa by more characters (“Orsten”), isolated and mounted on SEM stubs by the work− associated with the feeding and locomotory apparatus, such ing team of K.J.M. in Bonn, Germany in the 1980s to the mid as a fleshy labrum behind the original hypostome, fusion of 1990s. The holotype (UB 649, Fig. 1) of Oelandocaris oelan− the postoral sternites of the antennal to maxillulary segments dica is derived from the Olenus gibbosus Zone (lowermost (sternal plate, sternum), paragnaths on the mandibular por− Furongian Series, Peng et al. 2004; Babcock et al. 2005; tion of the sternum and the appearance of fine setulae or Terfelt et al. 2005) of the Alum Shale succession near Deger− denticles on setae, parts of the labrum, the sternum and the hamn, Öland, Sweden. The additional material is from the paragnaths (Maas et al. 2003). These features are also signifi− Agnostus pisiformis Zone at Gum, Västergötland, Sweden, cant in testing proposed atelocerate/tracheate or hexapod re− now representing the uppermost zone of the Middle Cambrian lationships (e.g., Wolff and Scholtz 2006 recognized homo− (Babcock et al. 2005). The additional material comprises six logy between the paragnaths of myriapods, hexapods and specimens, five larger ones (UB W 260–264, size range from crustaceans). Antennular morphology, development of an− more than 660 μm to ca. 1 mm, Table 2) and a considerably tenna and mandible, exopod morphology, and the occurrence smaller specimen (UB W 265), which is ca. 440 μm long. This and particularly the ontogenetic development of the “proxi− specimen shares characteristic features with the larger speci− mal endite” on postantennular limbs, may eventually facili− mens but evidently represents an earlier developmental stage. tate further resolution of the relationships among the crusta− A brief overview of the preservation of the individual speci− cean stem taxa. mens and features they display is given in Table 1. All compa− Re−investigations of Müller’s original set of taxa (Müller rable features among the specimens correspond in structure, 1983) on the basis of additional material allowed systematic topology, and size. Therefore the material is considered con− assignment of Skara anulata Müller, 1983, Bredocaris admi− specific with the holotype despite the stratigraphical and geo− rabilis Müller, 1983 and Rehbachiella kinnekullensis Müller, graphic distance between Öland and Västergötland. In the ab− 1983 to particular eucrustacean taxa (summary in Walossek sence of diagnostic differences, establishment of a new spe− and Müller 1998b). Walossekia quinquespinosa Müller, 1983 cies for the material from Västergötland is not tenable. and Dala peilertae Müller, 1981 (Müller 1981 treated this Due to the poor preservation of the holotype (Table 1), it species already in sufficient detail to qualify as a valid taxo− is difficult to interpret its morphology well, as exemplified nomic description, accordingly this date has to be used in by the misinterpretation of segmentation and a miscount of preference), awaiting detailed restudy, possess features limbs in the original description (Müller 1983, corrected in known only from particular crown crustaceans such as Stein et al. 2005). The new specimens, though better pre− cephalocarids, maxillopods and branchiopods, considered to served (Fig. 2), are considerably distorted and incomplete. form the monophylum Entomostraca (cf. Walossek 1999). However, the new material increases our understanding of Oelandocaris oelandica Müller, 1983 was originally the holotype,
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